GSA Bulletin Starts 2013 with 13 New Papers Published Online Ahead of Print

Boulder, Colorado, USA - GSA Bulletin papers published online 11–29 Jan. 2013 include contributions from scientists in the U.S., Canada, Mexico, Spain, the UK, and New Zealand. Multiple locations in the western U.S. are studied, along with locations in Canada, New Zealand, and Guatemala.

Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GSA Bulletin in your articles or blog posts. Contact Kea Giles for additional information or assistance. Non-media requests for articles may be directed to GSA Sales and Service,
.

Large subaqueous landslides and turbidites in Lake Tahoe suggest that strong shaking due to earthquakes (greater than magnitude 7) in or near the basin occurs every 800 years on average with the most recent event occurring about 500 years ago. This study by Shane Smith and colleagues provides the first continuous strong shaking record in the region spanning the Holocene and indicates that perhaps fourteen episodes have occurred during the past 12,000 years. Carbon-14 dating of the subaqueous deposits compares favorably to the record of known paleoseismic events on nearby faults. The size and location of the deposits also allows the researchers to determine when individual faults have ruptured. The West Tahoe fault, the largest fault in the basin, ruptured about 4,500, 5,600, and 7,800 years ago, giving a recurrence interval of about 2,600 years. The event about 4,500 years ago may not have ruptured the central and northern segments of the fault. More studies are needed to better define the seismic hazard in the Reno/Tahoe/Carson City region; however, these results confirm that the region has experienced major earthquakes throughout the Holocene.

The Cumberland basin is part of the large and deep Maritimes Sedimentary Basin of Atlantic Canada, interpreted to have developed at tropical latitudes, from about 350 to 300 million years ago, in the Mississippian and Pennsylvanian periods. The basin fill includes thick evaporites -- anhydrite, gypsum, rock salt. Overlying clastic sedimentary rocks include fossil forests with upright trees, preserved at the Joggins Fossil Cliffs UNESCO World Heritage Site. Analysis of seismic profiles demonstrates that the subsidence that allowed these strata to accumulate was produced by salt expulsion. In the western part of the basin, evaporites remained largely undisturbed until the Pennsylvanian Period, when their rapid expulsion allowed accumulation of the thick Joggins succession with its upright tree fossils. In the eastern part of the basin, evaporite withdrawal began earlier, producing "minibasins" filled by Mississippian strata. Many of the structures are similar to those on salt-bearing continental margins such as the eastern margin of North America. However, the tectonic environment encouraged vertical, rather than horizontal movement of salt. These features may be characteristic of salt tectonics in basins bounded by faults with horizontal movement. Salt expulsion has strongly influenced the distribution of hydrocarbons and other resources in the basin.

The Hornbrook Formation of southern Oregon and northern California preserves an important record of regional mid- to Late Cretaceous tectonic events that improves our understanding of paleogeography and regional tectonics for this critical time in the development of the North American Cordillera. Initial deposition in the Hornbrook basin likely began in a hanging-wall basin as footwall-block uplift exhumed the Klamath Mountains and generated a proximal sediment source for the lower members of the Hornbrook Formation. Kathleen Surpless and Emily Beverly suggest that changing plate motions along the U.S. Cordilleran margin during the Late Cretaceous shifted Hornbrook sediment sources to newly uplifted regions east and southeast of the basin, as continued subsidence occurred in the Klamath Mountains. Upper members of the Hornbrook Formation received sediment largely derived from the main Cretaceous Sierra batholith, with little to no sediment derived from the Klamath Mountains. A similar shift in sediment sources occurs in the Upper Cretaceous Great Valley Group of California, which likely formed the southern continuation of the Hornbrook basin during the Late Cretaceous. Thus, the record of changing sediment sources preserved within the Hornbrook Formation provides evidence of significant regional tectonic events in the mid- to Late Cretaceous Cordillera.

Slow-moving landslides pose a distinct natural hazard via damage to roads and structures, and can be significant drivers of erosion. Unlike conventional landslides, which fail catastrophically in a single event, these landslides creep at rates of centimeters to meters per year and can be continually active for years to centuries. Despite their importance, there has been little work predicting where slow-moving landslides occur. In this paper, Joel S. Scheingross and colleagues examine how tectonic faulting affects the distribution of slow-moving landslides. In a case study along the San Andreas fault, California, USA, they find that slow-moving are extremely common along the "creeping" section of the fault, which does not experience large earthquakes. However, slow-moving landslides are rare along the "locked" section of the fault, where large earthquakes are common. Their results suggest that reduced rock strength near the fault promotes the development of slow-moving landslides along the creeping San Andreas fault. In the locked section of the fault, however, large earthquakes induce rapid, catastrophic landslides which preferentially remove weak material from hillslopes and limit the development of slow-moving landslides. These results represent a first step in predicting the location of slow-moving landslides and are useful for hazards analysis and infrastructure planning.

Global change is accelerating the rate of sea-level rise and will likely affect the rate of intense hurricanes impacting coastlines in the future. This could dramatically impact sandy coastlines worldwide; however, there is a lack of field-based studies utilizing the geologic record of past periods of rapid sea-level rise and hurricanes to shed light on these potential future coastal changes. In this paper, Davin Wallace and John B. Anderson describe the evolution of a coupled Texas barrier island system (Galveston/Follets Islands, San Luis Pass Tidal Delta, and offshore) over the last few millennia by measuring erosion due to sea-level rise and hurricanes. Their data show that during periods of high sediment supply and rapid sea-level rise, the time-averaged volumetric erosion flux offshore due to hurricanes is higher. During periods of reduced sediment supply and slower sea-level rise, the time-averaged volumetric erosion flux offshore due to hurricanes is lower. They couple these data with the volumetric sand flux to a nearby tidal delta, which records the barrier’s erosion through time. Wallace and Anderson then show that historic erosion along the upper Texas coast is unprecedented relative to the geologic record and associate these changes with accelerated sea-level rise punctuated by hurricane events.

The southernmost portion of the continental North America Plate ends in central Guatemala, where it is known as the Maya block. There, Luigi A Solari and coauthors have studied the complex evolution of the Rabinal Granite, which first crystallized from a partial melting of sediment, and then got involved in a subduction-related metamorphism in the Late Cretaceous, during the transpressional collision with the Pacific Caribbean arc.

Jeff Munroe of Middlebury College and Ben Laabs of SUNY-Geneseo investigate the history of Lake Franklin, which occupied the Ruby Valley of northeastern Nevada during the last Ice Age. Although the lake has completely dried up, mapping of preserved beaches allowed the former dimensions of the lake to be determined, and radiocarbon dating of fossil shells recovered from beach sediments at a variety of elevations allowed a history of lake-level changes to be reconstructed. Lake Franklin was present during the global peak of the last Ice Age; however, it covered only ~50% of its maximum area, suggesting that Ice Age climate in northeastern Nevada was cold and dry. Approximately 17,000 years ago, the lake rose rapidly to its maximum elevation, expanding to cover 1,100 square kilometers, making it one of the largest lakes in the southwestern U.S. at that time. This rapid lake-level rise was synchronous with expansions of other lakes in the region, implicating a climatic shift toward moister conditions. Approximately 15,000 years ago, the level of the lake fell rapidly as the climate became abruptly drier. This trend was temporarily interrupted by a short-lived lake expansion about 13,000 years ago, after which the lake disappeared completely.

This paper by M. Keegan Raines and colleagues examines the sedimentary record of ancient drainage networks as they changed in response to early mountain building in the adjacent Cordillera. The study attempts to test several hypotheses used to explain observed changes in sediment composition and to unravel predictable patterns of sediment dispersal that may be found in similar basins. The changes in sediment composition, along with geochronology, petrography, and paleocurrent measurements, are interpreted to indicate at least two distinct sediment sources entering the early Alberta Basin. One source of sediment is interpreted to be from the adjacent mountain belt, while other sediments are interpreted to be far travelled recycled sediments. This study sheds light into a classic example of a foreland basin, revealing early stages of development that have been poorly understood.

In many settings, rivers alternate between carving wide valley bottoms (straths) and cutting narrow gorges over time, thereby creating longitudinally continuous paired bedrock strath terraces along valleys. Strath terraces are used ubiquitously in geomorphology and tectonics; however, how and why they form remain poorly understood. Noah Finnegan and Greg Balco focus on Arroyo Seco in the central California Coast Ranges, where they test hypotheses for strath terrace planation and subsequent strath terrace formation.

To better understand how climate affects bedrock river incision and long-term landscape evolution in the absence of tectonic forcing, James Menking and colleagues quantify differences in the longitudinal profiles of eroding bedrock channels across the Kohala peninsula on the northern tip of the Big Island of Hawai'i.

The Early Eocene Climatic Optimum was the warmest period in Earth's history since the demise of the dinosaurs at the end of the Cretaceous. This warm period was likely caused by increased atmospheric CO2, but the timescale of changes in terrestrial climate and the ecological impacts of those changes are poorly understood. Ethan Hyland and colleagues reconstruct the paleoclimatic and paleoenvironmental conditions of the Wind River Basin (western Wyoming, USA) during the Early Eocene and compare these conditions to records from other continental basins and from the ocean. Fossil soils (paleosols) preserved within the Wind River Formation show a rapid increase in atmospheric CO2 during the Early Eocene (about 51 million years ago) that resulted in rapid increases in temperature and precipitation, all of which contributed to a shift in local ecosystems (primarily vegetation). The rapidity of this peak event indicates that previously assumed longer timescale causes like volcanism and weathering were not the driver of these atmospheric changes, and instead suggests that these transient regional to global patterns of climate change resulted from short-term drivers such as increased continental lake or oceanic carbon ventilation.

The Hope fault is a major strike-slip fault in the northern South Island of New Zealand. Paleoseismic studies along the remote western part of the fault (Hurunui segment) have led to the estimation of timing for the two most recent, but prehistoric large earthquakes on the fault there. The characterization of these event ages, which occurred at about AD 1655–1835 and AD 1425-1625, was achieved using radiocarbon dates, soil, and tree ages in the upper Hurunui river valley. The occurrence of two large earthquake ruptures on the Hurunui segment during the last 600 years is consistent with previous slip rate-derived estimates of earthquake recurrence. The Hope fault is a well-segmented system. When the paleoseismic records from other pieces of the fault system are considered, e.g., Conway, Hope River, Hurunui segments and the Hanmer fault, the results show that each had ruptured during the last 120-360 years (AD 1650-1888) and that each had ruptured 2-3 times during the last 700-900 years. These results have significant implications for considering: (i) the role of stress triggering and fault interaction along major strike-slip faults, and (ii) seismic hazard in the northern South Island in light of recent destructive earthquakes in the Canterbury region.

Over the past 25 years, our understanding of the physical processes that drive volcanic eruptions has increased enormously thanks to major advances in computational and analytical facilities, instrumentation, and collection of comprehensive observational, geophysical, geochemical, and petrological data sets associated with recent volcanic activity. Much of this work has been motivated by the recognition that human exposure to volcanic hazard is increasing with both expanding populations and increasing reliance on infrastructure (as illustrated by the disruption to air traffic caused by the 2010 volcanic eruption in Iceland). Reducing vulnerability to volcanic eruptions requires a thorough understanding of the processes that govern eruptive activity. Here, Katharine Cashman and R. Stephen J. Sparks provide an overview of the current understanding of how volcanoes work, focusing particularly on the physical processes that modulate magma accumulation in the upper crust, transport magma to the surface, and control eruptive activity.